In the comics, yes. (Donald's also quite, uh, deceased thanks to Sigyn if my research is right.) In the movie universe I think it's safe to say they are two separate people at this point in time. Of course you should never say never because any good writer could figure out ways of writing out of any kind of situation. The thing that Thor comic book fans have to realize is that the movie universe is a totally different reality than the comic books. Yes, the writers can draw from the comics, but also the movies allow for a bit more freedom from convoluted continuities as well.

All we have is a name. Dr Don Blake, Jane's Ex Boyf may not look like the Don we know at all.

I completely understand the movieverse Thor is different and that's why I asked for your obvious reasons as to why you want Thor/Sif. I think we've all come to terms with the fact Happy and Pepper are not going to happen, Favreau doesn't even look like Happy, Jane's not going to be a Doctor, Odin isn't a malevolent hole and Thor isn't going to get a magic secret ID and practice medicine. They are doing a decent job at streamlining the movieverse into the central, most powerful relationships and tensions in the comics. The next one for Thor is clashing with his father over Jane and her mortality.

As for whether Jane's in Thor 2. The interview on the front page with Stellan seems to confirm her involvement in Thor 2. If Marvel are talking to Stellan's agent about Selvig, then Jane can't be far behind.